Crystal growth apparatuses or furnaces, such as directional solidification systems (DSS), Czochralski (CZ) method furnaces, and heat exchanger method (HEM) furnaces, involve the melting and controlled resolidification of a feedstock material, such as silicon or sapphire, in a crucible to produce an ingot or boule. Production of a solidified ingot from molten feedstock occurs in several identifiable steps over many hours. For example, to produce a silicon ingot by the DSS method, solid silicon feedstock is provided in a crucible, often contained in a graphite crucible box, and placed into the hot zone of a DSS furnace. Alternatively, to produce an ingot, such as a sapphire ingot, by the HEM method, solid feedstock, such as alumina, is provided in a crucible containing a monocrystalline seed (which comprises the same material as the feedstock but with a single crystal orientation throughout) placed into the hot zone of a solidification furnace. A heat exchanger, such as a helium-cooled heat exchanger, is positioned in thermal communication with the crucible bottom and with the monocrystalline seed.
The feedstock in either method is then heated to form a liquid feedstock melt (without substantially melting the monocrystalline seed in the HEM method), and the furnace temperature, which is well above the seed melting temperature (e.g., 1412° C. for silicon), is maintained for several hours to ensure proper melting. Once melted, heat is then removed from the melted feedstock, often by applying a temperature gradient in the hot zone, in order to directionally solidify the melt (e.g., from the unmelted seed) to form an ingot. By controlling how the melt solidifies, an ingot having greater purity than the starting feedstock material can be achieved, and in the case of the HEM method a crystalline material having a crystal orientation corresponding to that of the monocrystalline seed can be achieved, which can each then be used in a variety of high end applications, such as in the semiconductor and photovoltaic industries.
During heating and cooling, various instruments may be used to monitor the process in order to ensure proper operation and to allow for any necessary adjustments. In particular, since temperature is a fundamental parameter in many types of furnace operations, where reliable and continuous measurement of the temperature is essential for effective control of the operation, one such device used in furnaces is a pyrometer. A pyrometer is a type of thermometer used to measure typically high temperatures (e.g., using thermal radiation to determine the temperature of an object's surface). For example, in a crystal growth apparatus or furnace, a pyrometer may rely on detecting differences in emissivity between solid and liquid feedstock. Generally, since a heated object gives off electromagnetic radiation, there are two common types of pyrometers: the optical pyrometer (color-based) and the radiation pyrometer (infrared and/or visible light-based).
Notably, both types of pyrometers require visibility to the heated object during the process, and are thus typically mounted to the side of a furnace with a desired vantage point through a viewport. One challenge associated with this arrangement, however, is that current pyrometer mounting assemblies require complete disassembly in order to clean windows to the viewports. Cleaning of the windows is generally required prior to every run for each port (that is, there may be a plurality of ports). Also, particularly for side pyrometers, the windows may also require cleaning during certain stages of the run. This disassembly requirement potentially compromises accuracy and repeatability of the pyrometer readings.